The present invention relates to a semiconductor memory device having normal and standby modes and also relates to a semiconductor integrated circuit and a mobile electronic unit including a device of that type.
A semiconductor memory device called a static random access memory (SRAM) is basically made up of flip-flops, and is easy to use because such a device needs no refreshing. An SRAM can also operate at a high speed and allows a large operation margin. By taking advantage of all these beneficial features, SRAMs are often used as memories for mobile electronic units, for example. In addition, as transistors have their sizes further reduced over the last couple of years, mobile electronic units have also been downsized.
However, the smaller a transistor, the lower its break-down voltage. Accordingly, a transistor of a very small size should be operated with its operating voltage lowered. Furthermore, to enable a transistor to operate at a low voltage without sacrificing its operating speed, the threshold voltage of the transistor needs to be reduced. For that reason, small-sized mobile electronic units, including cell phones, which are normally driven by battery, use transistors with a low threshold voltage. However, if the threshold voltage of a transistor is too low, then the transistor with that low threshold voltage cannot be cut off completely, thus allowing some leakage current to flow. In that case, an increased amount of current is dissipated in vain in a standby mode.
A battery-driven mobile electronic unit of a small size is required to operate at a low voltage and with low power dissipation. As for a cell phone, in particular, it is one of the key features determining its market value how long the cell phone can hold its standby state. And to make the stand-by state as long as possible, the amount of current dissipated in the standby mode should be minimized.